The present disclosure relates to ultrasonic transducers. More particularly, the present disclosure relates to ultrasonic imaging and therapeutic devices.
Ultrasonic transducers are employed in a wide array of applications and industries, and are commonly employed in therapeutic and imaging devices.
Typically, an ultrasound transducer includes an active piezoelectric element that is attached to a backing, where the backing is made from a material that prevents spurious acoustic reflections from reaching the active element and interfering with its performance. The backing of an ultrasonic transducer is usually designed to be sufficiently thick to reduce spurious acoustic reflections to levels that are below the electrical noise floor of the system, or at least to levels that allow for an acceptable dynamic range of greyscale in the ultrasound image to differentiate tissue structures and to allow for sufficient contrast between tissue structures of similar acoustic impedance. In many cases, such as handheld ultrasound probes, there is little restriction imposed on the total thickness of a transducer stack, thus allowing the backing layer of the transducer stack to be thick in order to attenuate residual acoustic energy and maintain a short pulse response for the device.
However, if the ultrasonic transducer involves a thin backing layer, such that the acoustic energy that enters into the backing is not completely absorbed or attenuated, then acoustic energy that reflects off of the bottom surface of the backing will return to the piezoelectric layer with a sufficiently large enough amplitude to interfere with the device performance. This time-delayed energy thus generates a secondary signal or reverberation signal within the transducer stack. In transmit mode, the secondary pulse represents a trailing pulse behind the primary pulse propagating into the imaging medium. In receive mode, the secondary pulse creates a potential difference across the electrodes of the active layer that will also be detected by the receive electronics and will be an artifact in any reconstructed image.
For example, such artifacts can be problematic for high axial resolution ultrasound biomedical imaging devices, which require the use of ultrasound transducers that exhibit short time responses with minimal secondary pulses or stack reverberation that will reduce the acoustic dynamic range of the ultrasound image. High frequency clinical ultrasound (generally considered to involve frequencies greater than approximately 5 MHz and in particular greater than 10 MHz), in particular, has found significant use in minimally invasive imaging, such as intracardiac and intravascular applications. For these applications, ultrasound transducers are incorporated into a catheter or other device that can be inserted into a lumen or cavity within the body. This constrains the dimensions of the transducer stack and the volume of backing material that can be included in the transducer design.
A common practice for increasing ultrasound attenuation within a transducer stack, in order to avoid secondary signals and reverberation, is the introduction of scatterers into the backing. Scatterers help to partially break up the spatial coherence of the acoustic energy within the backing medium by inducing spatially variant and localized partial reflections of the propagating acoustic waves. The extent to which the scatterers have an effect in breaking up the coherence will depend on the size of the scatters relative to the wavelength of the propagating wave. For example, in some catheter applications, depending on the desired imaging frequency and the size constraints of the catheter, scattering is not necessarily sufficient to suppress reverberations.
In certain catheter applications in which the transducer is stationary relative to the housing that holds the transducer stack, sloped or angled surfaces, in either the backing or in the housing, may be employed to help to reflect the acoustic energy in different directions such that the path length of the acoustic energy in the backing is effectively increased or that the energy does not return to the piezoelectric active layer or both. However, in other catheter applications in which the transducer may move relative to its surroundings, the backing layer may be the only layer that can be employed to reduce the secondary pulses and reverberation within the transducer stack.
One approach to mitigate the effect of spurious reflections is to use a stack of multiple layers of materials with different acoustic impedance, in effect creating a one-dimensional acoustic grating structure analogous to optical gratings that are, for example, extensively used in fiber optics and telecommunications. This grating structure has a uniform cross section underneath the active area of the transducer stack. Achieving an acoustic grating frequency bandwidth that is wider than the bandwidth of the transducer stack itself requires several layers of several acoustic impedances. This complicates the fabrication of the grating structure and increases the required precision in achieving the desired layer thicknesses. This approach may also require a thickness that exceeds the space constraints of the stack in the first place. Expressed differently, a grating that is small enough to meet the size constraints for a catheter-based imaging transducer may cause the bandwidth of the primary signal/pulse to be substantially reduced, since the functional bandwidth of the grating may become narrower than that of the transducer itself.